Fish farming is conducted throughout the world and with entire stocks or species of fish being extinguished or substantially reduced because of overfishing, environmental concerns or otherwise, fish farming has taken on increased importance.
Such farming is generally conducted within the natural ocean environment. The netting used to hold and raise the fish is located at areas offshore, preferably in coves and the like where the netting and the fish are somewhat sheltered from storms, which storms can be damaging to the farm enclosures. In such an environment, the farms are subject to general sea conditions including current and tidal flows.
At certain times of the year, algae or phytoplankton often appear within the waters where fish are being farmed. In the areas around the northwest coast of the United States and the west coast of Canada, the algae begin to appear in late spring or early summer. The presence of algae within the fish pens is not desirable and fish mortality will increase if the algae is allowed to persist and is not reduced or eliminated.
One technique for keeping algae from contacting the fish is to connect tarpaulins to the walkways supporting the netting of the pens or cages and allow the tarpaulin to extend downwardly on the outside of the netting thereby preventing the flow of algae into the fish pens where it can contaminate the water within the cages. Where current flows are not a problem, the technique is satisfactory although the oxygen levels within the fish pens can become depleted after time and must be replenished, generally with the use of water circulated from the lower portions of a water column to the upper portions and thence to the water within the cages. This circulation is generally created by air injection into a series of vertical pipes which creates upwards flow and displaces the water within the cages when the water is released from the pipes over the water surface.
However, current flows in the vicinity of the fish pens are frequently a problem when tarpaulins are used. This is so because the area of the tarpaulins is necessarily large in order to ensure the containment and diversion of algae and plankton. The current acts over the entire area of the tarpaulins and creates a significant force on the tarpaulin which tends to lift the tarpaulins from their vertical or hanging positions assumed in calm water. The tarpaulins then angle away from their vertical position about their upper attachment points. When the tarpaulins are lifted by the current, the algae can then enter the netting because a certain area of the tarpaulins is no longer effective to maintain the water integrity of the fish cages.
To deal with this problem, weights were attached to the bottom of the netting and the tarpaulins are connected top and bottom to the netting. Thus, there will be a greater tendency for the nets and the attached tarpaulins to maintain their position in the vertical position under the force of current flow. However, even with weights of significant mass, a rise in current flow will still deflect the nets and attached tarpaulins significantly so that the use of weights is only useful for relatively small current flows. In addition, the tarpaulins may only be attached when algae is present. Otherwise, current flows carrying fresh oxygen for the fish, will no longer be able to do so. When the presence of algae is expected, the tarpaulins are connected to the netting manually by a diver or divers and, when the algae is no longer a problem, the tarpaulins are removed. Such procedures are labor intensive, time consuming and difficult.
A further difficulty associated with the aforementioned water circulation problem when tarpaulins are used is that the vertical pipe in which the water flows has its outlet located above the surface of the water in the fish cages. The air injected bubbles that are dispersed into the atmosphere from the outlet of the water pipe do not disperse any energy to the water within which the fish are being raised. Thus, the mixing action of the water and air mixture coming from the outlet of the water pipe is generally limited to the immediate vicinity of the pipe outlet. Since it is desirable to have the mixing and replenishment of the water take place so as to provide more oxygen and better mixing over a greater volume of water, the present technique used for water and oxygen replenishment is undesirable.